I think that Vorchas in co-op may need to be toned down from Mass Effect 2. They may not have much health, but they can regenerate all of it if left alone long enough. I'm thinking their "Y" control (XBOX 360) should send them into a rage mode and gain a little bit of armor just like in Mass Effect 2.
As for the Quarians, I'm adament they'll finally bring in the males to do some fighting.
Pheonix races may (and I know this has come up before) be Protheans. Though they may need to tone Dark Channel down a bit for co-op play. Otherwise, Prothean players will rack up quite a kill count and leave the rest of the team short on kills removing any source of fun (or competition if you roll that way) from co-op for the remainder of the team.
However, I really hope this DLC comes out
AFTER the Extended Cut DLC if it's legit. I want my little blue children and more closure to the end of Mass Effect 3.